Wet media mills, such as the ones described in U.S. Pat. No. 5,797,550 issued to Woodall, et al, and U.S. Pat. No. 4,848,676 issued to Stehr, are generally used to mill or grind relatively large quantities of materials. These rather large media mills are not generally suitable for grinding small or minute quantities. U.S. Pat. No. 5,593,097 issued to Corbin recognizes the need for milling small quantities, as small as 0.25 grams, to a size less than 0.5 micron to about 0.05 micron in terms of average diameter in about 60 minutes.
The media mill described in the Corbin patent comprises a vertically oriented open top vessel, a vertically extending agitator with pegs, a motor for rotating the agitator, and a controller for controlling the rotational speed. The vessel is a cylindrical centrifuge or test tube formed of a glass, plastic, stainless steel, or other suitable material having an inner diameter of between 10 to 20 mm. The media suitable is described as any non-contaminating, wear resistant material, sized between about 0.17 mm to 1 mm in diameter.
The particulates to be ground and the grinding media are suspended in a dispersion and poured into the vessel. The agitator, with the peg end inserted in the vessel, is spun. The Corbin patent also discloses that the pegs should extend to within between about 1-3 mm of the sides of the vessel to provide the milling desired in the shortest possible time without damaging the materials and producing excessive heat. To avoid splattering created by vortexing of the material during mixing, the top peg of the mixer is positioned even with the top of the dispersion. No seal or cover is deemed needed during mixing or agitation if this practice is followed.
The Corbin patent also discloses that its micro media can be useful for forming medicinal compounds, food additives, catalysts, pigments, and scents. Medicinal or pharmaceutical compounds can be expensive and require much experimentation, with different sizes and quantities. The Corbin patent discloses that the preferred media for medicinal compounds are zirconium oxide and glass. Moreover, pharmaceutical compounds are often heat sensitive, and thus must be maintained at certain temperatures. In this respect, the Corbin patent discloses using a temperature control bath around the vessel.
In the media mill of the type described in the Corbin patent, even if the vessel is filled to the top peg, however, the rotating agitator in the dispersion creates a vortex, which undesirably draws air into the dispersion and foams the dispersion. Moreover, the open top configuration draws in contamination, making the mill unsuitable for pharmaceutical products. The temperature-controlled bath could spill into the open top container and further contaminate the product.
There is a need for a micro or small-scale media mill that avoids these problems. The present invention is believed to meet this need.
The present invention relates to a small-scale or micro media-mill and a method of milling materials, such as pharmaceutical products. The present small-scale mill, which can be vertically or horizontally oriented, can use a dispersion containing attrition milling media and the product to be milled. The milling media can be polymeric type, such as formed of polystyrene or cross-linked polystyrene having a nominal diameter of no greater than 500 microns. Other sizes include 200 microns and 50 microns and a mixture of these sizes.
In one embodiment, the mill has a relatively small vessel having an opening, an agitator, and a coupling, and a rotatable shaft mounted for rotation about a shaft mount. The agitator is dimensioned to be inserted in the vessel through the opening. Specifically, the agitator can have a rotor and a rotor shaft extending from the rotor. The rotor shaft is connected to the rotatable shaft. The rotor is dimensioned to be inserted in the vessel with a small gap formed between an outer rotating surface of the rotor and an internal surface of the vessel. The coupling detachably connects the vessel to the shaft mount. The coupling has an opening through which a portion of the agitator, such as the rotor shaft, extends. The shaft mount seals the vessel opening to seal the dispersion in the vessel. A seal can be provided to seal the portion of the agitator or the rotor shaft while permitting the agitator to rotate. The rotatable shaft can be driven by a motor or can be a motor shaft of a motor, preferably a variable speed motor capable of 6000 RPM.
In one embodiment, the coupling can have a threaded portion for detachably mounting to the shaft mount and a flange portion for detachably coupling to the vessel. In another embodiment, the coupling is integrally formed with the vessel and has a threaded portion for detachably mounting to the shaft mount.
The mill can include a cooling system connected to the vessel. In one embodiment, the cooling system can comprise a water jacket. Specifically, the vessel comprises a cylindrical inner vessel and an outer vessel spaced from and surrounding the inner vessel. The inner and outer vessels form a chamber therebetween. The chamber can be vessel shaped or annular. A flange connects the upper ends of the inner and outer vessel. The outer vessel (jacket) has at least first and second passages that communicate with the chamber. The cooling system comprises the outer vessel with the first and second passages, which is adapted to circulate cooling fluid.
In an alternative embodiment, the vessel can comprise an inner cylindrical wall having a bottom and an open top and an outer cylindrical wall spaced from and surrounding the inner vessel. The inner and outer cylindrical walls are connected together so that an annular chamber is formed therebetween. At least the first and second passages are formed at the outer cylindrical wall and communicate with the chamber to pass coolant. The bottom extends radially and covers the bottom end of the outer cylindrical wall. The bottom can have an aperture that allows samples of the dispersion to be withdrawn. A valve can close the aperture. Alternatively, the bottom can have an observation window for observing the dispersion.
In another embodiment, the vessel can include at least one port through which the dispersion is filled. The vessel includes at least two ports through which the dispersion is circulated. In this respect, the cooling system comprises the ports on the vessel for circulating the dispersion. The vessel can be horizontally oriented.
The rotor can be cylindrical, and can have tapered end surfaces. In one embodiment, the rotor is dimensioned so that its outer periphery is spaced no larger than 3 mm away from an inner surface of the vessel, particularly when the dispersion contains attrition media having a nominal size of no larger than 500 microns. The spacing or the gap is preferably no larger than 1 mm, particularly when the dispersion contains attrition media having a nominal size of no larger than 200 microns.
In another embodiment, the cylindrical rotor can have a cavity and a plurality of slots that extend between an inner surface of the cavity and an outer surface of the cylindrical rotor. In another embodiment, the cylindrical rotor can have a plurality of channels extending to an outer surface of the cylindrical rotor. In another embodiment, the cylindrical rotor can have a plurality of passageways extending between the tapered end surfaces of the cylindrical rotor.
One method according to the present invention comprises providing a dispersion containing a non-soluble product to be milled and attrition milling media having a nominal size of no greater than 500 microns; inserting the dispersion into a cylindrical vessel; providing an agitator and a coupling that closes the vessel, the coupling having an opening through which a portion of the agitator extends, the agitator comprising a cylindrical rotor and a shaft extending therefrom, wherein the cylindrical rotor is dimensioned so that an outer periphery is no greater than 3 mm away from an inner surface of the cylindrical wall; inserting an agitator into cylindrical vessel and sealingly closing the coupling, wherein the amount of dispersion inserted into the vessel is so that the dispersion eliminates substantially all of the air in the vessel when the agitator is fully inserted into the vessel; and rotating the agitator for a predetermined period.
Another method according to the present invention comprises providing a dispersion containing a non-soluble product to be milled and attrition milling media having a nominal size of no greater than 500 microns; providing an agitator having a cylindrical rotor and shaft extending therefrom; inserting the agitator in a horizontally oriented cylindrical vessel and sealing the cylindrical vessel, the cylindrical rotor being dimensioned to provide a gap of no greater than 3 mm between an outer surface of the rotor and an inner surface of the vessel; providing at least one port through the cylindrical vessel and maintaining the port at a highest point of the horizontally oriented cylindrical vessel; filling the cylindrical vessel with the dispersion until the dispersion drives out substantially all of the air in the vessel; and rotating the agitator for a predetermined period.
The method further includes cooling the vessel by jacketing the vessel and flowing water between the jacket and the vessel. Another method comprises externally circulating the dispersion through a plurality of ports formed through the horizontally oriented vessel to thereby cool the dispersion or refresh the dispersion.